Conventionally, there has been widely known a mask blank comprising a halftone phase shift film (hereinafter referred to as a phase shift film) made of MoSiN, MoSiON, or the like. In the manufacture of such a mask blank, it is usual to form a phase shift film on a main surface of a transparent substrate using a single-wafer sputtering apparatus. In a normal single-wafer sputtering apparatus, a rotary stage on which a transparent substrate is to be placed is provided in the lower part of a film forming chamber and a target is disposed directly above the rotary stage. However, in the case where the normal single-wafer sputtering apparatus is used in the formation of the halftone phase shift film, there has been a problem that the thickness of the film on the outer peripheral side of the main surface of the transparent substrate tends to be relatively small compared to that on the center side thereof due to a shape of the main surface of the transparent substrate being rectangular. The phase shift film is required to simultaneously achieve a function of transmitting exposure light at a predetermined transmittance and a function of producing a predetermined phase difference between the exposure light transmitted therethrough and exposure light transmitted in air for a distance equal to the thickness of the phase shift film. If there is non-uniformity in the thickness distribution in the plane of the formed phase shift film, there is a possibility of the occurrence of variation in transmittance distribution in the plane or the occurrence of variation in phase difference distribution in the plane. When a phase shift film of a material containing oxygen or nitrogen is formed on a transparent substrate by DC sputtering using as a target material a material containing silicon such as MoSiN or MoSiON, since a nitride of silicon or an oxide of silicon has low conductivity, particles due to charge-up tend to be produced on a target surface. There is a possibility that these particles fall onto the transparent substrate located directly below the target surface and enter the phase shift film, thereby forming defects. That is, there is also a problem that the defect occurrence ratio increases.
In order to solve the unique problems that arise when such a rectangular mask blank film is formed by sputtering, use is made of a single-wafer sputtering apparatus disclosed in JP-A-2002-090978 (Patent Literature 1). In this sputtering apparatus, a target is disposed obliquely above a rotary stage, on which a transparent substrate is to be placed, so that both horizontal and vertical distances are ensured between the transparent substrate and the target (see FIG. 2). By forming a phase shift film on the transparent substrate using the sputtering apparatus of such a structure (sputtering apparatus of a so-called oblique-incidence sputtering type), it is possible to prevent the thickness of the film on the center side of the substrate from becoming relatively large and further to reduce defects due to charge-up of a target surface.
On the other hand, there has been a problem that when a transfer mask is manufactured from a mask blank having a thin film made of a material containing a metal and silicon, such as MoSiN or MoSiON, the light resistance of the thin film is not so high to exposure light irradiated onto the transfer mask. Further, the resistance is also not so high to a chemical liquid for use in a process of manufacturing the transfer mask from the mask blank or to a cleaning liquid for use in cleaning which is carried out for the completed transfer mask. Further, the thin film of this material tends to have a relatively large compressive stress. In order to solve these problems, for example, as disclosed in JP-A-2002-162726 (Patent Literature 2), a heat treatment is applied to a glass substrate formed with a light-semitransmissive film containing a metal, silicon, and nitrogen.
On the other hand, by applying a heat treatment in air or in a gas containing oxygen to a mask blank comprising a light-semitransmissive film containing a metal, silicon, and nitrogen, the light resistance can be improved. In the case where the heat treatment is applied to this mask blank, there has been a problem that if the mask blank is naturally cooled after the heat treatment, the in-plane variation in the optical properties of the light-semitransmissive film increases. In order to solve this problem, for example, as disclosed in JP-A-2006-323236 (Patent Literature 3), a treatment is carried out to forcibly cool a mask blank immediately after a heat treatment by the use of cooling means.